Relay valves according to the prior art control very large quantities of compressed air, for example for brake cylinders of a compressed air brake system of the vehicle, with very small quantities of compressed air. The relay valve thus accelerates the filling and purging of the brake cylinders.
For this, such relay valves usually have a relay valve piston acting as a control piston which can be pressurized on one side with control pressure via a control chamber, and on the other side with the working pressure provided by a consumer, via a pressure medium output.
An inlet valve is arranged between a filling chamber which can be connected to a pressure medium source via a pressure medium input, and a working chamber which can be connected to a consumer via the pressure medium output. An outlet valve is arranged between the working chamber and a purge chamber leading to atmosphere.
When the control chamber is pressurized with a control pressure, the relay valve piston moves into the working chamber. The movement of the relay valve piston first causes the outlet valve to close and then the inlet valve to open. In this way, a pressure is built up in the working chamber which counters the control pressure on the relay valve piston.
When the pressures acting against each other are in equilibrium, the relay valve piston is moved back against its original movement. The inlet valve closes without the outlet valve opening, so that the pressure in the working chamber and below the pressure medium output is maintained.
When the control chamber is purged, the relay valve piston is moved further against its original movement direction and the outlet valve opens until the pressure in the working chamber has fallen to a value at which the forces on the relay valve piston are again in equilibrium and the outlet valve closes again. On complete purging of the control chamber, the working chamber and hence also the downstream consumers are fully purged. A relay valve working in this way is known for example from EP 1 844 999 A1.
With such a working method of the relay valve, the pressure set at the pressure medium output stands in a design-induced ratio to the control pressure. If for example a 1:1 ratio of the relay valve is desired, a design is necessary in which the size of the active faces of the relay valve piston are the same on the control side and on the working side. This is conventionally achieved by a piston guide introduced into the housing cover. This piston guide in the housing cover must however be sealed against the relay valve piston, whereby machining of the housing cover is essential in order to keep the friction losses at the sealing elements as low as possible and to guarantee tightness.
These known relay valves are usually integrated, together with further components, in a valve device. The valve device may for example be an electronic air preparation device, an electromagnetic parking brake device or an axle modulator.